Technical Field
The present disclosure relates to thermoelectric materials, and more specifically, bismuth-antimony seleno-telluride thermoelectric nanocrystals and methods of forming the same.
Related Art
Thermoelectric devices are a promising but largely unrealized technology to improve energy efficiency by scavenging the latent energy in waste-heat across a wide range of applications. While small-scale devices for both Peltier cooling and Seebeck electricity generators have carved out niches in the marketplace, wide-spread adoption hinges on improving the conversion efficiencies (i.e., figure-of-merit ZT) of thermoelectric materials at or below current state-of-the-art material prices. Pnictogen chalcogenide (Sb2-xBixSe3-yTey) semiconductors are a widely-researched material and the current industry standard for making thermoelectric devices for applications from near room-temperature to medium temperatures up to 350° C. Nanostructured versions of the pnictogen chalcogenides are crucial for performance breakthroughs. However, current industrial-scale synthetic techniques for nanostructures rely on energy-intensive melt-spinning and/or ball-milling to produce the materials. Solution-phase synthesis of nanocrystalline bismuth telluride (Bi2Te3) and related phases are known, but their scalability is hindered by solvent choice, expensive precursors, extreme pH conditions, difficult preparation/separation/cleaning steps, or some combination of the above.
A polyol-based microwave technique has been used to successfully synthesize nanoplates with excellent thermoelectric properties. This batch process starts with the separate preparations of the pnictogen and chalcogen precursors in a high-boiling organic solvent, which are then mixed and subjected to microwave irradiation. The microwaves activates the reaction between the precursors, and the resulting sulfur-doped Sb2-xBixSe3-yTey alloys reached ZT values of greater than 1.1. However, this procedure proved unfeasible to scale due to the cost of the n-octylphosphine-chalcogen precursors, difficult handling of high-boiling solvents and issues disposing of the solvent wastes.
FIG. 1 shows a Pourbaix diagram of tellurium (Te). As shown, tellurium is predominantly insoluble in aqueous solutions and exists as either the element or as an oxide. In reducing environments, tellurium can be reduced to telluride or ditelluride. While soluble reduced tellurium has been used successfully to form Sb2BixSe3-yTey alloys, scaling the reaction to industrial volumes is challenging as a consequence of the limited stability of aqueous precursors containing divalent tellurium. Tellurides are only thermodynamically stable outside the range of water stability; tellurides will gradually oxidize back to more stable forms even in controlled environments. On exposure to air, a film of grey tellurium metal is immediately formed. This degradation of the soluble precursor is not amenable to large-scale production. Furthermore, in order to completely reduce the tellurium, a large quantity of hydride-class reducing agents are necessary, significantly adding to the cost of the reaction and to the amount of alkali impurities present in solution which are known to dramatically lower the Seebeck coefficient (a) of bismuth telluride alloys by increasing the carrier concentration (ni).
FIG. 2 shows a Pourbaix diagram of selenium (Se). Selenium (Se) cannot be oxidized to a soluble compound using hydrogen peroxide at any pH level, although the Pourbaix diagram in FIG. 2 indicates that it should be thermodynamically possible. This suggests that the selenium (Se) oxidation is kinetically unfavorable. Furthermore, similar to the chemistry of tellurium, in order to completely reduce the selenium, a large quantity of hydride-class reducing agents are necessary, significantly adding to the cost of the reaction and to the amount of alkali impurities present in solution which are known to dramatically lower the Seebeck coefficient (a) of bismuth telluride alloys by increasing the carrier concentration (ni).